1. Field of the Invention
This invention relates to a process for preparing novel polymers containing carboxyl groups. More particularly, this invention relates to a process for incorporating carboxylic acid groups into polymers by mixing the polymer and a carboxyl-containing copolymer in a mechanical mixing apparatus.
2. Prior Art
It is well known that as a result of the presence of carboxyl functionality in a polymeric structure, there is an improvement in the properties of the polymer, such as improved solubility properties, receptivity to dyes, adhesion to polymeric and non-polymeric substrates including metals, permeability to gases, interaction with fillers and reinforcing agents and the ability to form polymer alloys and composites.
Carboxyl-containing polymers may be prepared by the copolymerization of an ethylenically unsaturated carboxylic acid such as acrylic acid or maleic acid with a suitable comonomer. This method is limited to monomers which copolymerize with the unsaturated carboxylic acid and to processes involving catalysts which are not deactivated by the carboxylic acid. For example, carboxyl functionality cannot be incorporated by copolymerization into high density polyethylene and isotactic polypropylene since the preparation of these polymers involves organometallic catalysts.
Alternative methods for preparing carboxyl-containing polymers involve grafting and/or reaction of a polymer with an unsaturated acid. Thus, acrylic or methacrylic acid can be grafted onto polyethylene under ionizing radiation (U.S. Pat. No. 3,211,808) while maleic anhydride is grafted onto polypropylene in the presence of an organic peroxide (British Patent No. 1,086,839) and onto polyethylene in the presence of benzoyl peroxide or azobisisobutyronitrile in the presence of air (Gabara and Porejko, J. Polymer Sci., A-1, 5, 1539 (1967).
Carboxyl groups may be appended to an unsaturated polymer by reaction of the latter with maleic anhydride. A low molecular weight polyethylene containing olefinic linkages, prepared by thermal degradation of high molecular weight polyethylene, undergoes reaction with maleic anhydride in the melt or in solution (French Patent No. 1,346,533).
The incorporation of carboxyl groups in a polymer containing aromatic rings, e.g. polystyrene, may be accomplished by ultraviolet irradiation of a solution containing the polymer and maleic anhydride. As a result of the irradiation, the aromatic ring forms an adduct with the maleic anhydride (U.S. Pat. No. 3,214,416).
The addition of a solution of maleic anhydride in styrene to a polymer which contains labile or active hydrogen atoms, at an elevated temperature in the absence of a free radical catalyst, results in the formation of a carboxyl-containing polymer (U.S. Pat. No. 3,708,555).
Each of the foregoing methods for preparing a carboxyl-containing polymer involves the use of a monomeric unsaturated carboxylic acid or anhydride.
The formation of block and/or graft copolymers by mechanochemical methods is a well known art. Under applied shear, polymer chains are ruptured to generate free radical sites at the ruptured ends of the chains. Suitable shear forces are encountered when the polymer is subjected to mechanical deformation in masticating or mixing equipment such as a Banbury mixture, a Brabender Plasticorder, a rubber mill, a screw extruder or any other of the well known mechanical mixing equipment normally used in the processing of thermoplastic, elastomeric or thermosetting polymers. It is generally considered that when a blend of two polymers is subjected to the mechanical degradation which occurs upon application of shear forces, both chains undergo rupture and the resultant macroradicals combine with the formation of block copolymers. Graft copolymers are considered to be formed concurrently as a result of chain transfer reactions, followed by radical coupling or the reaction of macroradicals with unsaturation present in one or both polymers.
In order to rupture polymer chains it is necessary that the polymers be in a viscoelastic state when subjected to shear. In the case of elastomers, the reaction may be carried out at room temperature, e.g. cold milling, since they exist in the viscoelastic state under these conditions. In the case of rigid polymers, the viscoelastic state is attained at elevated temperatures, e.g. above the softening or melting point, or upon swelling with a solvent or monomer.
The melt viscosity and viscoeleasticity of a polymer are a function of molecular weight. The lower the molecular weight of the polymer the lower the melt viscosity and viscoelasticity. The rate of mechanical degradation is proportional to the molecular weight of the polymer, the plasticity and the rate of shear. The limit to which degradation proceeds, i.e. the molecular weight obtained on prolonged mastication, is dependent only upon the plasticity and is independent of the initial molecular weight, i.e. polymers below the limiting value are not degraded by mastication processes (R. J. Ceresa, Block and Graft Copolymers, Butterworths, 1962, pp. 72-73). For example, the experimental limiting molecular weight is 70,000 in the mastication of rubber at room temperature (R. J. Ceresa, ibid., p. 69). It has been considered that when low molecular weight polymers, i.e., polymers having a molecular weight and viscosity below the minimum required for chain rupture are blended with polymers having a molecular weight and viscosity above the minimum required for chain rupture graft copolymerization does not occur. It was believed, therefore, that both polymers being blended must undergo chain scission to effect grafting. Thus, in order to graft carboxyl group-containing polymers onto mono- or diolefin polymer chains it was believed necessary in all cases to use, as the grafting polymer, a polymer having a sufficiently high molecular weight and viscosity that it would undergo chain rupturing upon being subjected to shearing forces. This is unsatisfactory where it is desired to introduce carboxyl groups into the polymer to improve the solubility properties, receptivity to dyes, adhesive properties, etc. of the polymers without significantly modifying the basic characteristics and physical strength properties of the monoolefin or diolefin polymer. The molecular weight of the carboxyl group-containing polymer would be so high that considerably large quantities of carboxyl group-containing polymer would have to be grafted onto the mono- or diolefin polymer in order to incorporate a sufficient amount of carboxyl groups into the mono- or diolefin polymer to get the desired effect, with the consequence that the other properties of the backbone polymer would be significantly altered.
It has long been desired to incorporate carboxyl functionality into mono- or diolefin polymers by mechanical means without significantly altering the properties of the polymer.